Upgrade dask-gateway to 0.6.1

[jcrist]

Dask-Gateway version 0.6.1 has been released, which includes several new features Pangeo may be interested in:

• Adaptive scaling (e.g. cluster.adapt())
• Automatic shutdown of idle clusters
• Several performance and resiliency bugfixes

This release also fixes a bug in the helm chart that prevents smooth upgrades between versions (see dask/dask-gateway#150 for more information). As such, upgrading the chart will require manually deleting the previous dask-gateway deployment and installing the new version. From 0.6.0 on this issue has been resolved and upgrades should be smoother.